The present invention relates to fiber optical connections and, more particularly, to a structure and method for coupling a multiple channel fiber optic cable to a multiple channel Vertical Cavity Surface Emitting Laser (VCSEL) transmitter and a multiple channel Perpendicularly Aligned Integrated Die (PAID) receiver.
The invention seeks to construct a package for coupling a multiple channel fiber optic cable to a multiple channel Vertical Cavity Surface Emitting Laser (VCSEL) transmitter and a multiple channel Perpendicularly Aligned Integrated Die (PAID) receiver. The active surface of both the receiving and transmitting dies (hereinafter xe2x80x9coptoelectronic diesxe2x80x9d) are oriented perpendicular to the plane of the laminate package. The package can be soldered directly to an end user card with its cables plugged directly through the tail stock. In other words, the cable can exit from the card in a direction parallel to the plane of the card.
Other advantages of this design are:
1) it has integrated strain relief, a latching detent, and safety features;
2) it uses available processes and materials;
3) it comprises a plastic ball grid array (PBGA) laminate for high speed operation and low cost;
4) it has a two-part construction design with independent testing of each part to improve overall yield;
5) it allows two strategies for removing heat from the package;
6) it includes various incorporated features to minimize electrical cross-talk, radiated RF and susceptibility to external RF; and
7) it incorporates several features that serve to protect optical surfaces and electronic components from damage.
The development of this type of design has proven to be difficult, since solid state devices (often referred to either as dies or chips), with active components on one side only, are usually mounted parallel to the card, with their optically active features perpendicularly oriented to receive or emit light. It becomes necessary, therefore, to provide means for orienting the optoelectronic chips perpendicular to the card so that the emitted or received light enters the package parallel to the card while maintaining the profile (height above the card) low enough to meet specified limitations imposed by the end user.
Among the features incorporated into the design are:
1) an overmolded ball grid array (BGA) that strengthens and stiffens the relatively weak BGA laminate. (This is needed because the carefully aligned optics, which must be held in place, are integrated into the package);
2) a package having integrated strain relief for the cable/connector with direct mechanical coupling to the card, to prevent disturbing the optical portions;
3) use of relatively low cost materials, assembly procedures and standard processes;
4) separate grounds within the BGA, a two part lid, a shroud and pins on the carriers for minimizing:
a) cross-talk from receiver to transmitter,
b) radiated RF power, and
c) susceptibility to RF pickup;
5) incorporation of many standard features of overmolded packages to minimize cost and susceptibility to damage;
6) an improved assembly/test strategy (to keep yields high and costs low); and
7) a dual path strategy for removing heat from the package to keep the optoelectronic parts cool enough for high speed operation.
There have been a number of attempts to develop a package and/or product that meets the general requirements of a parallel fiber optic link. A package developed for the JITNEY project, a government funded package developed at IBM by M. S. Cohen et al., xe2x80x9cPackaging Aspects of the Jitney Parallel Optical Interconnect,xe2x80x9d 1998 ECTC, pp. 1206-1215; and J. Crow et al., xe2x80x9cThe Jitney Parallel Optical Interconnect,xe2x80x9d 1996 ECTC, pp. 292-300, consisted of separate transmitter and receiver modules and two separate cables to complete a bi-directional optical link. The cable contained 20 fibers for the simultaneous transmission of 20 channels of information at the full data rate. This design permitted the transmission of two bytes of information, along with four bits of overhead, in each bit time. The transmitter module contained a driver chip (differential inputs) and a VCSEL transmitter. Both chips were mounted on a heatsink. The heatsink and chips were parallel to the module and the card.
A specially designed array lens served to redirect the light emitted from the VCSEL (perpendicular to the card) into the input face of the fibers which were disposed parallel to the card. Similarly, the receiver module contained a receiver chip (generating differential outputs) also oriented parallel to the module. The same array lens was used to redirect the light emerging from the optical fibers into the direction perpendicular to the card, so that the light to be detected impinged upon the surface of the receiver substantially perpendicular to the photosensitive surface of the receiver. Although several similarities may be found in the JITNEY package, it did not attempt to solve the crosstalk problem (transmitter to receiver). It used two separate modules. JITNEY had a separate strain-relief, used a leadframe (instead of a BGA), and generally did not use packaging techniques expected to support data transmission rates of 1 Gigabit per second per channel or more.
Two versions of link transceiver modules developed for the Motorola Optobus project are described in two papers: xe2x80x9cCharacteristics of VCSEL Arrays for Parallel Optical Interconnects,xe2x80x9d 1996 Electronic Components and Technology Conference (ECTC), pp. 279-291; and xe2x80x9cOptobus I: A Production Parallel Fiber Optical Interconnect,xe2x80x9d 1997 ECTC, pp. 204-209. Both modules have in common a number of packaging features: 1) a surface emitting VCSEL array is used, the light path is parallel to the host card plane, and the optoelectronic component is mounted perpendicular to the module""s BGA laminate, 2) a molded plastic waveguide structure conducts the light to/from the optoelectronic die to endfaces of the fibers in ribbon optical cables, the cables being terminated with MT connectors, 3) a glob encapsulated, multi-chip pin grid array laminate board is provided on which the optoelectronic subassembly is mounted, and 4) the resulting package is nonhermetic.
As fabricated, the loss of the low-loss waveguides is some tenths of a dB/cm. To meet safety goals and to increase the amount of optical power reaching the detector, the waveguides for the transmitter and for the receiver portions of this transceiver are not constructed identically. On the transmitter side, the waveguide is designed to increase the numerical aperture of the entering beam as the light passes from the VCSEL to the optical fiber. On the receiver side, the waveguide is designed to improve coupling efficiency from the optical fiber to the photodetector. A passive alignment procedure (i.e., the optical elements are not electrically activated during the procedure) is used to align the array of optically active elements on the optoelectronic dies to the molded structure which contain an array of waveguides.
In the Optobus transceiver (the 1996 paper), a leadframe for delivering the electrical signals to the optoelectronic die is overmolded and serves as the supporting structure for the waveguide arrays. Electrical connections are made from finished ends of the leadframe conductors, a nonstandard packaging approach, to contact pads on the top surface of the optoelectronic dies. The other end of each of the leadframe""s conductors, which are routed to exit points along the side of the molded waveguide structure and then bent down to attachment points, are electrically connected to pads on the top surface of the laminate board.
In the optobus I transceiver (the 1997 paper), a tape automated bonding (TAB) leadframe is used to replace the electrical function of the standard leadframe in the earlier version. The conductors on one end of this TAB leadframe make electrical contact to the optoelectronic dies and, on the other end, to contacts on the top surface to the laminate board. The TAB leadframe is bent 90 degrees between the two ends. Again, alignment between the molded waveguide structure and the optoelectronic dies is accomplished using a passive alignment technique.
The PAROLI (Siemens) project, xe2x80x9cParallel Optical Link for Multichannel Gigabit Rate Interconnections,xe2x80x9d H. Karstensen et al., 1988 ECTC, pp. 747-754, comprised an optical coupler which redirected the light 90 degrees. An array of multimode fibers was captured in a transfer-molded holder. The end of the fiber array was polished at an angle, and the optically active chips were die bonded in position below the polished facets, so that the light followed the desired path. Active alignment was used to position the chips in the facets. The optical coupler mated to an MT type of optical connector. Nonhermetic packaging was used.
To make the link, two separate 12-channel modules were used, one for the transmitter and one for the receiver. An AC coupled link was built with a data rate of 1 Gbit/s for each channel; a DC coupled link was built with a data rate of 500 Mb/s for each channel.
A PARABIT (NTT) 40-channel parallel optical interconnection module with a throughput in excess of 25 Gbit/s, K. Katsura et al., xe2x80x9cPackaging for a 40 channel Parallel Optical Interconnection Module with an Over 25-Gbit/s Throughput,xe2x80x9d 1998 ECTC, pp. 755-761, comprised a transceiver consisting of 20 transmitting and 20 receiving channels in one module, and utilized a multimode fiber. A 250 mm pitch was used, with 850 xcexcm VCSELs and GaAs pin photodiodes. Light was carried to and from the optically active chips by means of polymeric waveguides, which were cut at 45 degrees in order to redirect the photon path. Passive alignment of the chips to the waveguides was used, where the principle of index alignment was employed. A unique xe2x80x9cbare-fiberxe2x80x9d connector was used to couple the waveguides to the fibers. There, bare fibers protruded from the end of the connector. The fibers were inserted into micro capillaries, which were mated to the polymeric waveguides. The fibers were buckled to create a constant force on the ends of the waveguides, in order to maintain good physical contact.
In the POLO 980 nm DuPont project, by K. Hahn et al., xe2x80x9cGigabyte/sec Data Communications with the POLO Parallel Optical Link,xe2x80x9d 1996 ECTC, pp. 301-307, a bottom-emitting VCSEL array chip was used with a PIN photodiode. These chips were incorporated into two widely spaced sub-modules, each of which had 10 channels. Each channel was projected to operate at 1 Gbit/s. The optical coupler which transmitted light to and from the MT connector was based on polymer waveguides, and consisted of commercially available xe2x80x9cPolyguidexe2x80x9d (DuPont) waveguides. A 90xc2x0 redirection in the light path was accomplished by providing a 45xc2x0 bevel in the end of the polyguide, thereby forming a mirror. The optically active chips were first diebonded, before the polyguide was aligned.
A Hitachi package by A. Miura et al., xe2x80x9cReliable, Compact, CMOS Interface, 200 Mbit/sxc3x9712-channel Optical Interconnects Using Single-Mode Fiber Arrays,xe2x80x9d 1997 ECTC, pp. 225-230; and A. Takai et al., xe2x80x9c200 Mb/s/ch 100-m Optical Subsystem Interconnections Using 8-Channel 1.3-mm Laser Diode Arrays and Single-Mode Fiber Arrays,xe2x80x9d J. Lightwave Tech., vol. 12, pp. 260-270, 1994, comprised 200 Mb/sxc3x9712 channel optical interconnects using single-mode fiber arrays. The Hitachi modules, which are separate transmitter and receiver modules, were designed for long wavelength single-mode operation. For this purpose 1.3 xcexcm edge-emitting lasers were used, together with full hermetic sealing. Planar microlens arrays were employed for light coupling. An array of 12 channels was used for both the transmitter and receiver modules. Each channel ran at 200 Mbit/s. The fibers were placed in silicon V grooves for accurate placement. Alignment was accomplished by first roughly aligning using stereo microscope image processing, and accurate adjustment was then accomplished using computer controlled, two-dimensional scanning.
A 3.5 Gbit/sxc3x974 ch Interconnect (NTT), by N. Tanaka et al., xe2x80x9c3.5 Gb/sxc3x974 ch Optical Interconnection Module for ATM Switching System,xe2x80x9d 1997 ECTC pp. 210-216, provided a structure for a multichannel fiber optic package using silicon V-groove technology to provide a 4-channel fiber array with the fibers encased in micro-capillaries. The fiber ends were hemispherically lensed. Passive alignment was used by bonding the edge-emitting laser to the same silicon substrate which held the fibers. The same principle was used for the photodiode array, but the fiber lenses were slant polished in this case. In both cases a fiber pitch of 250 mm was used. Each channel was operated at 3.5 Gbit/s.
A one Gbyte/s Array (NEC), by T. Nagahori, xe2x80x9c1-Gbyte/sec Array Transmitter and Receiver Modules for Low Cost Optical Fiber Interconnection,xe2x80x9d 1996 ECTC pp. 255-258, comprised silicon V grooves used in conjunction with edge emitting lasers. The laser was passively aligned to the silicon substrate by solder-bump technology, as was the monitor photodiode. No lenses were used there. The light was redirected 90 degrees to the photodiode by etching a slope on the silicon submount and metallizing. A 1.3 xcexcm wavelength was used. Eight channels operating at 200 Mb/s each gave a 1 Gb/s throughput.
In the POINT project (GE, Amp, Honeywell, Allied Signal), Y. S. Liu et al., xe2x80x9cPlastic VCSEL Array Packaging and High Density Polymer Waveguides for Board and Backplane Optical Interconnect,xe2x80x9d 1998 ECTC, pp. 999-1005, a plastic VCSEL array package was provided with high density polymer waveguides, for a board and backplane optical interconnect. POINT was a demonstration project to show feasibility of low cost VCSEL and receiver array packaging. There, a GE process was used to attach the optically active chips to a polymer film on which conductor lines had previously been defined. Next, epoxy encapsulant was introduced in order to impart good mechanical stability to the structure. A polymer film was fabricated over the chips. This polymer film was patterned by a precision laser micromachining system in order to fabricate passive alignment features into the film. These alignment features were keyed to fiducial marks on the chips. The alignment features were used for passive alignment of polymer waveguides to the optically active chips. The system permitted 10 channels each for the transmitter and receiver sides of the package. The modules were apparently not given full functional tests.
Another package was developed by OETC (Lucent, IBM, Honeywell, Univ. Minnesota, Univ. Illinois), Y. M. Wong et al., xe2x80x9cOptoelectronic Technology Consortium Parallel Optical Data Link: Components, System Applications, and Simulation Tools,xe2x80x9d 1996 ECTC , pp. 269-278.
Yet another package (Hewlett Packard Laboratories and Univ. of North Carolina) was described by P. Rosenberg et al. in a paper entitled xe2x80x9cThe PONI-1 Parallel-Optical Linkxe2x80x9d, 1999 Electronic Components and Technology Conference, IEEE, pp. 763-769. The PONI devices separated the receiver and transmitter functions, and did not disclose a transceiver device, nor a faraday barrier, which permits the close proximity of the transmitting and receiving functions. No means was disclosed to allow the pins to be prealigned before final accurate engagement occurs. The PONI device permitted pin engagement of an MT connector directly to a metal heatsink/base. The PONI alignment scheme was xe2x80x9cpassivexe2x80x9d, in that electrically active optoelectronic components were not energized to assist in the alignment process. An electronic chip was mounted close to the optoelectronic device, both chips mounted perpendicularly relative to the board on which the PONI package was mounted, significantly raising the height profile of the overall package. Because the chips were located in close proximity to one another, heat removal from the sensitive optoelectronic part was problematical, since the more heat tolerant electronic device required heat removal, as well. That is, removing heat from the relatively heat tolerant device had to be accomplished without damaging the more sensitive optoelectronic part.
Some of the differences of the current invention over the aforementioned art are as follows:
The package of this invention does not use polymer films which serve as waveguides anywhere in its construction. The use of optically transparent media spanning the distance from the optoelectronic chip to the optical coupler is anticipated, but this is not a light guiding structure.
The invention uses an array of fiber stubs to form the optical coupler.
The invention does not use silicon bench (silicon V groove) technology in its module except, possibly, in the construction of the optical coupler portion of the module. Silicon bench technology may be used in the fabrication of the ferrule portion of the connector used to terminate the array of fibers in the cable.
The invention does not use self-aligning solder-bump attachment techniques within the module.
The invention is a transceiver module, using surface emitting rather than edge emitting lasers, and does not use microlenses.
The invention does not use leadframes.
The preferred fabrication approach uses active alignment.
In U.S. Pat. No. 5,420,954, issued to Swirhun et al on May 30, 1995 for PARALLEL OPTICAL INTERCONNECT, an optical interconnect that couples multiple optical fibers to an array of optoelectronic devices, is illustrated. The patent describes a parallel connection between the chip and the fiber cable. The connection of these parts are very close, but it is not desirable to have a removable part so close to sensitive electronic parts.
In U.S. Pat. No. 5,818,994, issued to Hehmann, on Oct. 6, 1998 for DEVICE FOR THE UNADJUSTED COUPLING OF A NUMBER OF OPTICAL WAVEGUIDES TO A LASER ARRAY, a coupling of optical fibers to a laser array is shown. The patent teaches a non-removable connection between the optoelectronics and the optical fiber array.
In U.S. Pat. No. 5,832,150, issued on Nov. 3, 1998 to Flint, for SIDE INJECTION FIBER OPTIC COUPLER, a coupler is illustrated that couples an asymmetrical beam of a laser diode into a fiber optic cable. The device depicts a reflective end face to reflect laser radiation. The input facet is approximately parallel to the central axis of the array.
In U.S. Pat. No. 5,631,988, issued on May 20, 1997 to Swirhun et al for PARALLEL OPTICAL INTERCONNECT, an optical interconnect is shown that couples multiple optical fibers to an array of optoelectronic devices with a parallel orientation.
In U.S. Pat. No. 5,121,457, issued to Foley et al on Jun. 9, 1992 for METHOD FOR COUPLING LASER ARRAY TO OPTICAL FIBER ARRAY, a method is disclosed of using V-grooves to align individual fibers in precise relationship to their light emitting devices. The fibers have polished end face facets at forty-five degrees, which are orthogonally attached to a mating configured surface.
In U.S. Pat. No. 5,454,814, issued on Nov. 12, 1996 to Noddings et al for PARALLEL OPTICAL TRANSCEIVER LINK, an optical interconnect module is shown that mates with an optical fiber connector having a parallel orientation. The link comprises a sapphire window having metallized features for conducting electronic signals to a VCSEL.
In U.S. Pat. No. 5,781,682, issued to Cohen et al on Jul. 14, 1998 for LOW COST PACKAGING FOR PARALLEL OPTICAL COMPUTER LINK, a coupling apparatus is shown for coupling a connector of a parallel optical cable to a receiver or transmitter array.
In U.S. Pat. No. 5,774,614, issued on Jun. 30, 1998 to Gilliland et al for OPTOELECTRONIC COUPLING AND METHOD OF MAKING SAME, a coupling is depicted that provides an optoelectronic device to be attached and aligned with a flexible substrate whose end face is mounted to an optical waveguide, similar to that shown for the present invention. The flexible waveguide provides the ability to orient the connection. The present invention departs from this patent by incorporating strain relief latching, RF isolation, reduction in electrical cross-talk, a heatsink, and fabrication with a protective overmolding.
In U.S. Pat. No. 5,432,630, issued on Jul. 11, 1995 to Lebby et al for OPTICAL BUS WITH OPTICAL TRANSCEIVER MODULES AND METHOD OF MANUFACTURE, a transceiver module is shown which includes both semiconductor devices and optoelectronic devices, as well as coupling features for attaching a parallel cable. The optical coupler is an array of molded waveguides, not an optical coupler made of an array of fibers. No overmold is used, nor are details provided regarding how strain relief or any prealignment function is accomplished. No disclosure of back side electrical contact to the light generating or light receiving devices is made. No means are provided by which heat is removed from the various heat generating elements. This would be important, especially at high operating speeds. No flexible circuit is disclosed for conducting current from the top surface of the laminate to the optoelectronic chips.
In accordance with the present invention, there is provided a single package for coupling a multiple channel fiber optic cable to a multiple channel Vertical Cavity Surface Emitting Laser (VCSEL) transmitter and for coupling a second multiple channel fiber optic cable to a multiple channel Perpendicularly Aligned Integrated Die (PAID) receiver. The active surface of both the receiving and transmitting (optoelectronic) dies are oriented perpendicular to the plane of the laminate package. The package can be soldered directly to an end user card, and have its cable plugged directly through the tail stock. In other words, the cable can exit from the card in a direction parallel to the plane of the card.
The package article comprises a laminate table or board (hereinbelow referred to simply as a laminate) upon which amplifier dies, preferably attached by wirebonds or other attachment means known in the art, are supported. The laminate carries an overmold frame that houses, optionally, a faraday barrier shield for RF isolation purposes. The overmold frame supports an optical subassembly which accepts an optical connector that is attached to an end of the parallel fiber optic cable. A retainer substantially encloses an optical coupler. Attached to the optical coupler is a heatsink carrier which, in turn, supports an optoelectronic die. One function of the heatsink carrier is to remove heat from the optoelectronic die. The heat drawn into the heatsink carrier may be dissipated into the nearby air. Optionally, the heat may pass through a heat conducting compound to a package cover where it is then dissipated to the air.
Electrically attached to the optoelectronic die is a flexible circuit. The flexible circuit itself can be electrically attached to the heatsink carrier. The flexible circuit is mechanically supported by the heatsink carrier using an adhesive. One purpose of the flexible circuit is to reorient the electrical signals from pads on the surface of the laminate to pads on the optoelectronic die, which is substantially perpendicular to the plane of the laminate.
Optoelectronic elements on the optoelectronic die are aligned with the optical coupler and end faces of the optical fibers in the connectors, so that optical signals pass from the optoelectronic elements to the optical fibers. The space between the optoelectronic die and the nearby end face of the optical coupler is optionally filled with a substantially transparent material. This transparent material also serves as a weak mechanical link between these two elements. Shock forces applied to the optical coupler are thus prevented from adversely affecting the optoelectronic die. The transparent material also assists in protecting the optoelectronic die from environmental sources of contamination.
An optical coupler subassembly consists of the optical coupler, the heatsink carrier, the optoelectronic die and the flexible circuit. The optical coupler is secured to the heatsink carrier on one distal end thereof. The optical connector, secured to one end of a parallel fiber optic cable, is partially contained within a receptacle in the retainer and held by a latch mechanism against the other distal end of the optical coupler. The optical coupler comprises an array of optical fibers mounted substantially within a protective housing. The end faces of the protective housing and the optical fibers are prepared so that the end faces of the optical fibers have an optical finish.
The optical subassembly consists of the optical coupler subassembly attached to the retainer. Specifically, the optical coupler portion of the optical coupler subassembly fits into a receiving bore in the retainer. The retainer has mechanical features which serve to align it to similar complementary features in the overmold frame. In a preferred embodiment, the retainer and optical coupler subassembly are adhesively attached to each other.
The package article can be a transceiver on a single laminate for receiving and transmitting.
It is an object of this invention to provide an improved coupling assembly between a horizontally oriented fiber optic cable, and a vertically oriented optoelectronic die.
It is another object of the invention to provide a method for accepting signals from a host card and generating corresponding high-current signals for transmitting to an optoelectronic die; and for accepting low-current signals generated by the optoelectronic die and amplifying and digitizing the received signals to levels suitable for transmitting to the host card.
It is yet another object of the invention to provide a transceiver that has a vertically oriented die and that accepts a horizontally oriented parallel fiber optic cable, wherein forces applied by the optical connector to externally accessible parts of the package article are substantially mechanically isolated from the optoelectronic die.